1. Field of the Invention
The present invention relates to an ophthalmologic image pickup apparatus used in an ophthalmologic doctor's office or the like.
2. Related Background Art
Up to now, there has been known an apparatus that picks up an image of an eye to be examined using an image pickup element, which is represented by a CCD, to convert the image into an image signal, thereby performing observation and image pickup on the eye to be examined. In particular, when performing infrared fluorescent image pickup by using a fundus camera, a fundus cannot be observed through an optical finder. Therefore, alignment and focusing are performed using the above-mentioned image pickup element.
In general, a phase of contrast in the fluorescent image pickup is classified into an initial phase of contrast, a middle phase of contrast, and a later phase of contrast. The initial phase of contrast is a first period from the start of choroid contrast to the completion of contrast of a choroid vein. The middle phase of contrast is a subsequent period from the completion of contrast of the choroid vein to the loss of a fluorescent agent from the choroid vein. The later phase of contrast is a final period during which a diffuse choroid background fluorescence is observed.
In the initial phase of contrast, a fluorescent agent injected (hereinafter merely referred to as “infused”) into a vein of a person to be examined reaches a thick blood vessel of a fundus by blood circulation. As the phase of contrast becomes the middle phase of contrast and then the later phase of contrast, the fluorescent agent gradually penetrates a thin blood vessel with the lapse of time. Therefore, a concentration of the fluorescent agent present in a blood vessel in the initial phase of fluorescent contrast becomes much higher than the concentration of the fluorescent agent present in the blood vessel in the later phase of fluorescent contrast. Thus, because of the circulation of the fluorescent agent, a fluorescent illuminance of the eye to be examined in the initial phase of contrast is much higher than the fluorescent illuminances of the eye to be examined in the middle phase of contrast and the later phase of contrast. In addition, a change in illuminance in the initial phase of contrast becomes larger than the changes in illuminance in the middle phase of contrast and the later phase of contrast.
In general, a dynamic range of a fluorescent illuminance of a fundus is very wider than the dynamic range of an image pickup element. Therefore, in either cases where the fluorescent illuminance is low or the fluorescent illuminance is high, it is hard to control an observation light intensity, an image pickup light intensity, a gain of the image pickup element, and the like, with the result that it is difficult to perform image pickup with preferable image quality.
In order to solve such a problem, there has been proposed a method using an auto gain control (AGC) capable of obtaining a constant image signal, even if the fluorescent illuminance of the eye to be examined has been changed. In this method, when fluorescent observation is performed, the AGC is operated and controlled so that an average value of an image signal from an image of the eye to be examined is kept constant, even if the brightness of the eye to be examined, that is, the observation light intensity has been changed. On the other hand, when fluorescent image pickup is performed, a light emission time of an image pickup light source is as short as several milli-seconds. Even if the AGC is operated, the gain cannot be followed. Therefore, to constantly obtain a proper image of the eye to be examined, a gain of an image signal from the image pickup element is changed from the AGC to a fixed gain at image pickup timing and the image pickup light intensity is controlled according to the brightness of the eye to be examined.
Also, there has been proposed an image pickup method as described in Japanese Patent Application Laid-Open No. H02-124137. According to the image pickup method, a timer for detecting an elapsed time from the infusion of a fluorescent agent is provided. When a signal indicating the elapsed time from the timer is received, an image pickup light emission intensity is increased with an elapsed time from the infusion.
The method using the AGC according to the above-mentioned conventional examples is effective to the case of initial image pickup for the fundus observation, in which the fluorescent illuminance at the fundus is high. However, in the later phase in which the fluorescent illuminance value significantly reduces, it is necessary to prepare a high sensitive image pickup element or an observation light source having a large light emission intensity. Therefore, there is a disadvantage in that a size of an apparatus becomes larger or a cost of a system becomes higher. In addition, when a sensitivity of the image pickup element is not sufficiently high, the AGC is not operated. Therefore, there is another disadvantage in that a gain at a time when the AGC is changed to the fixed gain and a proper exposure value of the image pickup light intensity cannot be obtained in the image pickup.
In the method as described in Japanese Patent Application Laid-Open H02-124137, a fluorescent illuminance at the fundus, which changes with a lapse of time significantly varies according to personal differences such as age, sex, weight, and height, an infused fluorescent agent amount, an infusion rate, a disease of a patient, or the like. Therefore, in the method of increasing the image pickup light intensity based on only the elapsed time from the infusion, there is a disadvantage in that a halation in the initial phase and a poor contrast in the later phase cannot be avoided.